I know that you could use Paint to create solid blocks of colour on the tents. Whether you could keep the shading, I am not sure.
If you want tents for knights there are some brightly coloured (red, yellow, blue and white) ones in Paperboys "Castle Assault – Sieges and Battles Edward I to Bannockburn":
link
Paperboys also have Marching Camp Roman tents which look like leather. Some are simple ridge tents in brown leather and some are larger Officers or Headquarters tents in red leather. These are in their book "Roman Invasion AD 43 – 84":
link
There may be others in some of their other books, since I do not have every book in the series. You can buy hard copy books or cheaper electronic downloads.
Amongst the downloads, for only £1.00 GBP, there are these:
link
Lastly, if you want 19th Century tents, PaperTerrain do some, both Bell tents and Marquees. They can be seen here:
paperterrain.com/tents
Rod